I. Field of the Invention
This invention relates generally to cardiac rhythm management in the treatment of patients exhibiting congestive heart failure (CHF), and more particularly to an apparatus and method for measuring total acoustic noise (TAN) during each heart beat or acoustic noise measured in predetermined defined segments of a cardiac cycle and utilizing the measured value to optimize the pacing mode for a given CHF patient.
II. Discussion of the Prior Art
Early on in the development of so-called rate-adaptive pacemakers in the treatment of bradycardia it has been recognized that an accelerometer can be incorporated into the implantable pacemaker device for sensing physical activity so that the pacing rate can automatically be adjusted between upper and lower limits to accommodate the patient's hemodynamic demand. That is to say, an activity sensor in the form of a piezoresistive bridge can be used to produce an electrical control signal that varies with the level of physical activity of the patient. More recently, cardiac stimulating systems have been designed for implantation in patients suffering from CHF and various algorithms have been devised for optimizing the mechanical performance of the heart as a pump by appropriately adjusting the AV delay parameter of the pacemaker. In this regard, reference is made to the Lawrence C. Baumann, et al. application Ser. No. 08/815,697, filed Mar. 12, 1997, now U.S. Pat. No. 5,836,887 and entitled "Apparatus and Method for Optimizing the Cardiac Performance by Determining the Optimal Timing Interval from an Accelerometer Signal" as well as the V.A. Kadhiresan application Ser. No. 09/009,424, filed Jan. 20, 1998 and entitled "Long Term Monitoring of Acceleration Signals for Optimization of Pacing Therapy", both of which are assigned to applicant's assignee and their contents incorporated by reference herein.
Studies conducted by applicant's assignee, in which accelerometer signals from CHF patients were collected and analyzed, confirm that the heart generates acoustic energy (sound) as a by-product of its normal function. The character of this energy, in terms of its frequency spectrum, amplitude, morphology, etc., is determined by preload, after-load, contractility and cardiac chamber timing and, thus, is affected by factors, such as the atrio-ventricular (AV) interval and pacing site(s) (right chamber, left chamber or both chambers or multiple sites in any chamber). The analysis of accelerometer signals from CHF patients has revealed that the accelerometer signals increased in amplitude during exercise and increased linearly with patient workload, and that under fixed conditions, the overall accelerometer amplitude decreases as the pacing parameters approach optimum values, as defined by aortic pulse pressure or the peak positive rate of change of left ventricular pulse pressure with respect to time (LVdP/dt). LVdP/dt is a known measure of the heart's contractility. Upon noting the above relationships in data sampled from a number of CHF patients, we defined a parameter TAN, an acronym for Total Acoustic Noise, as the high pass filtered and rectified accelerometer signal integrated over a cardiac cycle for each heart beat. TAN values were derived by high pass filtering the raw accelerometer output signal with a filter having a corner frequency of about 1.0 Hz to remove baseline variations due primarily to respiration. Next, the high pass filtered accelerometer signal was rectified to produce a monopolar signal and that signal was integrated over each cardiac cycle.
The use of the TAN measure was first envisioned as a rate-adjusting parameter in a rate-adaptive pacemaker. It was noted that the amplitude of the accelerometer signal increases with exercise and is monotomically related to work load as well as to peak aortic blood velocity. Based on those relationships. It was originally thought that during a pacing optimization protocol that TAN would be maximized at the optimum pacing site and AV interval. This would seem reasonable in that it would coincide with the maximum cardiac output or pulse pressure. Instead, we found that aortic pulse pressure was a maximum when TAN was a minimum. In the same way that optimum tuning of an automobile engine results in minimum generated noise, so too does optimum AV interval result in a minimum TAN value at any given work load.
The Kieval patent 5,554,177 teaches that in cardiomyopathy, there is a particular abnormal heart sound which is minimized during an optimization process. Using the automobile engine analogy, this would be the equivalent of detecting an engine "knock" and then adjusting the timing to reduce its amplitude. The method of the present invention relies upon TAN and is predicated on the fact that all of the acoustic energy generated by the heart during its entire cycle is artifact and that optimization of AV timing reduces the overall level of "noise". The approach of the present invention has the advantage of greater simplicity over the system described in the Kieval '177 patent. It does not require that a particular objectionable acoustic component be identified, but instead, all of the acoustic signal generated by the heart can be integrated and the pacing mode modified so as to minimize the total acoustic noise. The TAN approach of the present invention is simpler and easier to implement. Since it is not necessary to identify S1 and S2 heart sounds, the processing power required is significantly reduced.
FIG. 1 is a plot of percentage change in TAN versus percentage change in LVdP/dt and FIG. 2 is a similar plot of aortic pulse pressure, each for both a left ventricular pacing site and for biventricular pacing sites measured during a pacing optimization protocol. These plots show that LVdP/dt (contractility) and pulse pressure maxima occur at the minimum TAN value, but also there is a linear negative relationship between the parameters. Our studies allow the conclusion to be drawn that the pacing site (LV pace or BiV pace) and the AV interval which will generate the maximum peak positive LVdP/dt and maximum aortic pulse pressure can be determined from the minimum TAN value.
Based upon the discovery that TAN is a minimum when an optimization protocol maximizes aortic pulse pressure and the heart's contractility, as evidenced by left ventricular dP/dt, a new algorithm has been developed for optimizing the hemodynamic performance of the heart in CHF patients.